From the Latin meaning summit, the apex is the tip of a pyramidal or rounded structure, like the lung or the heart. The apex of the lung is indeed its tip, its rounded most superior portion. The apex of the heart is likewise its tip, but that is formed by the left ventricle so it is essentially the most inferior portion of the heart.
APEX
Meaning of APEX in English
English dictionary of medicine. Английский словарь медицины. 2012